PEX Tubing questions

I am planning on laying down some PEX tubing in about 2 weeks for in my garage. From what I've been told, I want the 1/2" oxygen barrier tubing. What brands are any good? I have heard different stories on the lengths of the runs. I am currently planning 250 foot runs. Does that sound alright? I doubt that I will have an operational system (heat) by the time the cement is poured. Would there be a problem if I didn't have an operational system when the cement is poured?

Thanks for your help.
Craig